Hay fever is not caused by hay and does not cause a fever.
“Hay fever” is a commonly used term for seasonal allergic rhinitis, which can produce such symptoms as runny nose, itchy eyes and throat, uncontrollable sneezing, and sometimes itching of the skin. Some people have an over-active immune system which identifies normally harmless particles, such as pollens or animal dander, as dangerous. This causes an excessive reaction that actually causes inflammation — an allergy. The substances causing it are allergens.